The Southern Ocean plays an essential role in hemispheric and global ocean circulation, biogeochemical cycles, and climate. Complex interactions occur between the atmosphere, ocean, cryosphere and marine organisms, with significant impacts on the carbon cycle, regional ecosystems, future climate change and sea level rise. The Southern Ocean is a critical sink for anthropogenic carbon dioxide, but recent reports have suggested that this sink may be weakening, due to changes in its overturning circulation. This remains a topic of active debate. The Southern Ocean is warming more rapidly than the global ocean as a whole, due to mechanisms that are not yet fully determined. Progress requires long-term observations, but the Southern Ocean is remote and hostile, and the paucity of comprehensive datasets makes hypotheses regarding the impact of Southern Ocean processes on the adjacent ice sheet, ecosystems and climate difficult to validate. Recent technological developments are beginning to give unprecedented insight into the role of the Southern Ocean in the global system, including autonomous floats and gliders, animal-borne sensors, new and more accurate satellite systems, and refined numerical models. This session aims to bring together modellers and observers to assess the current state of knowledge concerning the role of the Southern Ocean in the global climate system, and to exchange ideas concerning how to further such understanding.